idzio: Prepare for switchable backends

idzio/backend.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <stdint.h>
+
+#include "idzio/idzio.h"
+
+struct idz_io_backend {
+    void (*jvs_read_buttons)(uint8_t *gamebtn);
+    void (*jvs_read_shifter)(uint8_t *gear);
+    void (*jvs_read_analogs)(struct idz_io_analog_state *state);
+};